The present invention relates to a method of driving an electroluminescent device (EL device) configured such that a layer having a light emitting region is provided between an anode and a cathode.
Cathode-ray tubes (CRT) having high luminance and high color reproducibility have been most widely employed as displays; however, they are bulky and heavy, and high in power consumption. On the other hand, lightweight, highly efficient flat panel displays have been actively studied and developed, for example, for picture display of computers and television sets.
For example, liquid crystal displays of an active matrix driven type or the like have been put on the market as lightweight, highly efficient flat panel displays.
Liquid crystal displays, however, have problems that the viewing angle is narrow, the power consumption of a back light is large under a dark environment because of the use of no spontaneous light, the response to high-speed video signals of high definition is insufficient although such a response characteristic is expected to be put to practical use in future, and an after-image phenomenon occurs. In particular, liquid crystal displays with large screen sizes have a further problem that the production costs are raised because of difficulties in producing such large-sized liquid crystal displays.
It may be considered to use displays incorporating light-emitting diodes in place of CRTs. Such displays, however, are also high in production costs and have difficulties in forming a matrix structure of light-emitting diodes on one substrate. Accordingly, displays of this type have a problem to be solved before the displays are put to practical use as low-cost substitutes for CRTs.
In recent years, organic electroluminescent devices (organic EL devices) using organic luminescent materials have become a focus of interest as flat panel displays capable of solving these problems. These organic EL devices using organic compounds as luminous materials are expected as flat panel displays of a type which makes use of spontaneous light, exhibits high response speeds, and have no dependence on viewing angle.
Organic electroluminescent devices are configured by forming an organic thin film, which contains a luminescent material that emits light by injection of a current, between a translucent anode and a metal cathode.
C. W. Tang and S. A. VanSlyke have disclosed a device structure (an organic EL device having a single-hetero structure) in Applied Physics Letters, Vol. 51, No. 12, pp. 913-915 (1987). This device structure has a double-layered structure including an organic thin film made from a hole transport material and an organic thin film made from an electron transport material, wherein light emission occurs by re-combination of holes and electrons injected from respective electrodes into the organic thin films. In this device structure, either of the hole transport material or the electron transport material serves as a luminescent material, and emits light in a wavelength band corresponding to an energy gap between a ground state and an excited state of the luminescent material. The organic EL device using such a double-layered structure is advantageous in significantly reducing a drive voltage and improving the luminous efficiency.
Since then, C. Adachi, S. Tokita, T. Tsutsui, and S. Saito have disclosed a three-layered structure (organic EL device having a double-hetero structure) in Japanese Journal of Applied Physics, Vol. 27, No. 2, pp. L269-L271 (1988). The three-layered structure has three layers made from a hole transport material, a luminescent material, and an electron transport material. C. W. Tang, S. A. VanSlyke, and C. H. Chen have disclosed a device structure that a luminescent material is contained in an electron transport material in Journal of Applied Physics, Vol. 65, No. 9, pp. 3610-3616 (1989). These researches have suggested that light emission with high luminance at a low voltage may be possible, and on the basis of such suggestion, studies have been very actively made to develop new type organic EL devices.
Organic compounds used as luminescent materials are advantageous in that since such organic compounds have a diversity in kind, the color of light emitted from a light emitting layer can be freely changed in theory by selecting an organic compound, which has a molecular structure suitable for emission of light of a desired color, as a luminescent material forming the light emitting layer. Accordingly, as compared with thin film EL devices using inorganic devices, it is easier to obtain organic EL devices having devices allowing emission of light of reds (R), green (G), and blue (B) with good color purities necessary for full-color display.
Metal complex materials containing organic ligands coordinated around metal ions, which are not organic materials in the strict sense of the word, have been used as electroluminescent device materials. Such metal complex materials generally belong to the category of organic EL device materials.
A typical example of the metal complex material includes tris(8-quinolinol)aluminum with three molecules of 8-quinolinol coordinated around an aluminum ion. This metal complex material is hereinafter referred to as “Alq3”. The light emission of Alq3 is due to fluorescence from the π-π* excited state locally present in the coordinated 8-quinolinol molecules, and therefore, it may be regarded as light emission from an organic matter.
In recent years, along with diversification of materials, complex materials, which have a structure that ligands are coordinated around a transition metal and a rare earth metal, wherein light emission occurs by MLCT (metal-to-ligand charge transfer) or LMCT (ligand-to-metal charge transfer), have become a focus of interest of luminescent materials.
Like the above-described organic compounds used as luminescent materials, metal complex materials used as luminescent materials are advantageous in that since such metal complex materials have a diversity in kind, the color of light emitted from a light emitting layer can be freely changed in theory by selecting a metal complex material, which contains organic ligands having a combination and molecular structures suitable for emission of light of a desired color, as a luminescent material forming the light emitting layer.
In recent years, study and development using phosphorescence in place of fluorescence from the viewpoint of improvement of luminous efficiency have become noticeable. In the case of using a metal complex, since an atomic mass of a center metal is large and its atomic cloud is spread, a probability of intersystem crossing between excited states is increased, which leads to expectation for light emission from a triplet excited state lower than a singlet excited state, that is, phosphorescence.
High molecular EL devices using high molecular materials as luminescent materials have been extensively developed. As compared with a low molecular material, a high molecular material is advantageous in relatively easily forming a light emitting layer not by a vacuum process but by a coating process or a printing process, and is therefore expected as a material capable of shortening a tact time, and reducing an investment for a production apparatus. These high polymer luminescent materials may generally have a structure similar to that of conductive high polymers, and therefore, they are greatly advantageous in allowing the drive of an organic EL device including a luminescent layer made from a high polymer luminescent material at a low voltage.
In general, an organic EL device emits light when a current is applied only in one direction, that is, a voltage having one polarity is applied between electrodes, and therefore, it functions as a diode rectifier allowing the flow of a current and emission of light only by a forward bias. Accordingly, a waveform of a perfect direct current/voltage or a pulse with a single polarity is used for driving a light emitting device.
Such a direct current drive or a single polarity pulse drive of an organic EL device is liable to cause device deterioration for the following reasons (1), (2) and (3).
(1) Polarization of migratable ion species, which are contained as impurities in a device structure, in a single layer or between electrodes, to incidentally or inevitably deteriorate the device.
(2) Ionization of an element originally contained in an electrode material by application of an electric field, followed by elution of the element as migratable ions in the device structure, to change the electric field state of a layer structure precisely designed, thereby causing device deterioration.
(3) Decomposition of an organic material in an excited state.
On the other hand, to realize stability and sustainability of a light emitting device, there have been proposed a drive method by an asymmetric type AC drive with voltage control (Japanese Patent Laid-open No. Hei 8-180972) and a bipolar type drive method (Published Japanese translation of a PCT application No. Hei 11-500574).
The former drive method, however, has a problem that since an organic EL device is driven by current control with its luminous intensity specified, if a resistance change occurs, it fails to sufficiently cope therewith. In other words, such a drive method is disadvantageous in that the condition of optimization of an asymmetric waveform is unclear. The former drive method is intended to cause light emission in band regions of both polarities of a bipolar waveform, and if a device has a substantially ideal device structure with the enhanced luminous efficiency, a diode rectifying characteristic is improved; however, since light emission occurs at the time of reverse bias, the device may be significantly deterioration.
A method of suppressing device deterioration by applying a voltage in the direction reversed to that of a DC voltage between an anode electrode and a cathode electrode has been disclosed in Japanese Patent No. 3169974; however, this document does not sufficiently examine the optimization of the magnitude and timing of the voltage in the direction reversed to that of the DC voltage.